1. Field of the Invention
The present invention relates generally to automatic gain selection circuits, and more particularly to a column amplifier having automatic gain selection for CMOS image sensors.
2. Description of the Related Art
In order to design improved CMOS image sensors for low light sensitivity, excellent low noise performance is desired. Besides the noise generated directly in each pixel, the noise contribution of all subsequent analog processing stages must be considered as well. To minimize the impact of these system noise sources, it is beneficial to amplify each pixel sensor signal as early as possible in the image sensor. Early amplification leads to an improved signal-to-noise ratio, since the signal is amplified, but the noise of the system components after the amplification stage is not. The problem with signal amplification, however, is that large signals, when amplified, can end up being outside of the dynamic range of the sensor In other words, signal amplification reduces the over-all dynamic range of the system.
One prior attempt to solve this problem is disclosed by S. Kawahito et al., A Column-Based Pixel-Gain-Adaptive CMOS Image Sensor for Low-Light—Level Imaging, Proc. of IEEE International Solid-State Circuits Conference (ISSCC), 2003, San Francisco, Volume 46, Page 224. As shown in FIG. 1, a comparator compares the pixel signal directly with a reference signal, in order to determine whether or not to amplify the signal. If the signal is below a certain threshold, the signal is amplified, otherwise it is not. However, there are many non-uniformities from pixel to pixel in an image sensor, making it difficult to select an appropriate threshold for the amplification decision, or requiring a unique threshold for each pixel.
Therefore, it would be desirable if a selective amplification could be applied to the pixel signals in an image sensor, wherein the amplification decision is less dependent on the non-uniformities of the pixels.